1. Field of the Invention
The present invention relates to an exposure apparatus and a device fabrication method.
2. Description of the Related Art
An exposure apparatus is employed to fabricate semiconductor devices such as a semiconductor memory and a logic circuit using a photolithography technique. The exposure apparatus projects and transfers a pattern formed on a reticle (mask) onto a substrate such as a wafer by a projection optical system.
When the exposure apparatus starts exposure, the projection optical system partially absorbs exposure light, so its temperature changes and its optical characteristic, in turn, changes. Also, the pattern of the reticle generally includes a pattern (V pattern) having the vertical direction as its longitudinal direction, and a pattern (H pattern) having the horizontal direction as its longitudinal direction. Therefore, when the V pattern and the H pattern have different pitches, the change in temperature of the projection optical system resulting from factors associated with exposure light varies between the light path of light from the H pattern and that of light from the V pattern. As a result, the shapes and internal refractive index distributions of a lens and mirror of the projection optical system change due to the difference between the change in temperature in the light path of light from the H pattern and that in the light path of light from the V pattern, thus generating astigmatism.
Japanese Patent Laid-Open No. 11-145054 discloses a technique for reducing the adverse effect of astigmatism. The technique disclosed in Japanese Patent Laid-Open No. 11-145054 will be described with reference to FIG. 8. FIG. 8 is a graph showing the relationship between the cumulative exposure time (abscissa) and the focus positions (ordinate) of light from the H pattern and light from the V pattern. As the cumulative exposure time increases, the focus position Meas_H of light from the H pattern and the focus position Meas_V of light from the V pattern change due to a change in temperature of the projection optical system resulting from factors associated with exposure light, as shown in FIG. 8. Hence, Japanese Patent Laid-Open No. 11-145054 determines the average position between the focus position Meas_H of light from the H pattern and the focus position Meas_V of light from the V pattern as a focus position in exposure, thereby reducing defocus resulting from astigmatism.
However, when the average position between the focus position of light from the H pattern and that of light from the V pattern is determined as a focus position in exposure, this average position falls outside the depths of focus of these patterns. If, for example, the H pattern has an occupied area larger than the V pattern on the reticle, the rise in temperature in the light path of light from the H pattern is greater than that in the light path of light from the V pattern. In this case, a difference occurs between the amount of change in focus position of light from the H pattern and that of light from the V pattern, so the above-mentioned problem becomes especially serious. Also, in general, the pattern actually transferred onto the substrate has a depth of focus which differs depending on the pattern direction. Therefore, a shift occurs between the focus position optimum for exposure and the average position (the focus position in exposure) between the focus position of light from the H pattern and that of light from the V pattern.